1. Field of the Invention
The present invention relates to an organic electroluminescent display panel device and method of fabricating the same, and more particularly, to an organic electroluminescent display panel device and a method of fabricating the same that has a high aperture ratio and high definition images.
2. Discussion of the Related Art
In general, an organic electroluminescent device emits light by injecting electrons from a cathode electrode and holes from an anode electrode into an emissive layer, combining the electrons and the holes to generate an exciton, and transiting the exciton from an excited state to a ground state. Since the organic electroluminescent device is self-luminescent and does not require an additional light source, the organic electroluminescent device has a small size and is light weight, as compared to a liquid crystal display device. The organic electroluminescent device also has low power consumption, high brightness, and short response time. Thus, the organic electroluminescent device is used in many consumer electronics, such as cellular phones, car navigation systems (CNSs), personal digital assistants (PDAs), camcorders, and palm PCs. In addition, the organic electroluminescent device can have reduced manufacturing costs because of its simple manufacturing processes.
Organic electroluminescent devices may be categorized into passive matrix-type and active matrix-type depending upon how the device is driven. Compared to an active matrix-type, passive matrix-type organic electroluminescent devices have a simpler structure and are fabricated through a simpler manufacturing process. However, the passive matrix-type organic electroluminescent devices have higher power consumption, thereby preventing use in large area displays. Furthermore, in passive matrix organic electroluminescent devices, aperture ratio decreases according to the increasing number of electrical lines. Thus, the passive matrix-type organic electroluminescent devices are commonly used as small-sized display devices. In contrast, active matrix-type organic electroluminescent devices are commonly used as large-sized display devices since they have high luminous efficacy, and provide high definition images.
FIG. 1 is a cross sectional view of an organic electroluminescent display panel device according to the related art. In FIG. 1, the organic electroluminescent device 10 includes a first substrate 12 and a second substrate 28, that face each other with a predetermined space therebetween. A plurality of thin film transistors T and a plurality of first electrodes 16 are formed on an inner surface of the first substrate 12, wherein each of the first electrodes 16 are connected to each of the thin film transistors T, respectively. In addition, organic luminescent layers 18 are formed on the first electrodes 16 and the thin film transistors T, and a second electrode 20 is formed on the organic luminescent layers 18. The organic luminescent layers 18 emit light in one of three colors: red (R), green (G), and blue (B) within a pixel region P, and are generally formed by patterning an organic material.
A desiccant 22 is formed on an inner surface of the second substrate 28 to remove any external moisture and air that may permeate into a space between the first and second substrates 12 and 28. The inner surface of the second substrate 28 is patterned to form a groove, and the desiccant 22 is disposed within the groove and is fastened with a tape 25.
A sealant 26 is formed between the first and second substrates 12 and 28, and surrounds array elements, such as the thin film transistors T, the first electrodes 16, the organic luminescent layers 18, and the second electrodes 20. The sealant 26 attaches the first and second substrates 12 and 28 together and forms an airtight space to protect the elements from the external moisture and air.
FIG. 2 is a plane view of a pixel of the organic electroluminescent display panel device of FIG. 1. In FIG. 2, the pixel includes a switching thin film transistor (TFT) TS, a driving thin film transistor (TFT) TD, and a storage capacitor CST. In addition, a gate line 32 and a data line 34 are formed on the first substrate 12, and are formed of a transparent material, such as glass and plastic. The gate line 32 and the data line 34 cross each other, thereby defining the pixel region P, and a power line 35 is formed parallel to the data line 34.
The switching TFT TS includes a gate electrode 36, an active layer 40, a source electrode 46, and a drain electrode 50. The driving TFT TD includes a gate electrode 38, an active layer 42, a source electrode 48, and a drain electrode 52. In particular, the gate electrode 36 of the switching TFT TS connects to the gate line 32, and the source electrode 46 of the switching TFT TS connects to the data line 34. The drain electrode 50 of the switching TFT TS connects to the gate electrode 38 of the driving TFT TD through a first contact hole 54, and the source electrode 48 of the driving TFT TD connects to the power line 35 through a second contact hole 56. The drain electrode 52 of the driving TFT TD connects to the first electrode 16 in the pixel region P. A capacitor electrode 15 overlaps the power line 35 to form the storage capacitor CST, and is made of doped polycrystalline silicon and connects to the drain electrode 50 of the switching TFT TS.
FIG. 3 is a layout of the organic electroluminescent display panel device of FIG. 1. In FIG. 3, a display area is defined in a central region of the first substrate 12. A data pad portion E is formed in an upper side of the first substrate 12, and a first gate pad portion F1 and a second gate pad portion F2 are formed in left and right sides of the first substrate 12, respectively. A common electrode 39 is formed in a lower side of the substrate 12. The common electrode 39 applies a common voltage to the second electrode 20, which functions as a cathode electrode and is formed over the display area, and maintains the common voltage.
FIG. 4A is a cross sectional view along IVA—IVA of FIG. 2. In FIG. 4A, the driving TFT TD is formed on the substrate 12, and includes the gate electrode 38, the active layer 42, and the source and drain electrodes 48 and 52. The storage capacitor CST is formed over the substrate 12 and is parallel connected to the driving TFT TD. The storage capacitor CST includes the capacitor electrode 15 and the power line 35, which function as a first capacitor electrode and a second capacitor electrode, respectively. The capacitor electrode 15 is made of polycrystalline silicon. An insulating layer 57 covers the driving TFT TD and the storage capacitor CST, and the first electrode 16 is formed on the insulating layer 57 to electrically contact the drain electrode 52. An organic luminescent layer 18 that emits one color of light is formed on the first electrode 16, and the second electrode 20 is formed on the organic layer 18.
FIG. 4B is a cross sectional view along IVB—IVB of FIG. 3. In FIG. 4B, the common electrode 39 is formed in a side of the substrate 12 to apply a common voltage to the second electrode 20 (FIG. 4A). The common electrode 39 may be made of the same material as the gate electrode 38 of the driving TFT TD (FIG. 4A). The common electrode 39 is exposed by a first common contact hole 60 and a second common contact hole 62 through insulating layers. The second electrode 20 connects to the common electrode 39. An input line (not shown) from the outside could connect to the common electrode 39 through the second common contact hole 62.
A yield of the organic electroluminescent device depends on yields of the thin film transistor and the organic layer. Especially, the yield of the organic electroluminescent device varies due to impurities in the process of forming the organic layer to a thickness of about 1,000 Å. Accordingly, the yield of the organic electroluminescent device of the related art is reduced because of the impurities, thereby resulting in a loss of manufacturing costs and source materials for the thin film transistor.
Moreover, the organic electroluminescent device of the related art is a bottom emission mode device having stability and degrees of freedom for the manufacturing processes. However, the bottom emission mode device has a reduced aperture ratio. Thus, the bottom emission mode organic electroluminescent device has difficulty in being used as a high aperture device.
On the other hand, a top emission mode organic electroluminescent device has a high aperture ratio, and is easy to manufacture. However, in the top emission mode organic electroluminescent device, since a cathode electrode is generally disposed over the organic layer, a choice of material with which to make the cathode electrode is limited. Accordingly, transmittance of light is limited, and a luminous efficacy is reduced. Furthermore, in order to improve light transmittance the passivation layer should be formed as a thin film, whereby the exterior moisture and air is not fully blocked.